The present invention generally relates to polymeric silicone compositions useful as a contact lens material.
Polymeric silicone materials have been used in a variety of biomedical applications, including, for example, in contact lenses and intraocular lenses. Such materials can generally be subdivided into hydrogels and non-hydrogels (the latter referred to herein as "low water" materials). Silicone hydrogels constitute crosslinked polymeric systems that can absorb and retain water in an equilibrium state and generally have a water content greater than about 5 weight percent and more commonly between about 10 to about 80 weight percent. Such materials are usually prepared by polymerizing a mixture containing at least one silicone-containing monomer and at least one hydrophilic monomer. Either the silicone-containing monomer or the hydrophilic monomer may function as a crosslinking agent (a crosslinker being defined as a monomer having multiple polymerizable functionalities) or a separate crosslinker may be employed.
Low water silicone materials may include rigid materials, useful for rigid gas permeable contact lenses, or flexible materials useful for soft contact lenses. Like their hydrogel counterparts, such low water silicone materials may be based on the same class of silicone-containing monomeric units; however, unlike silicone hydrogels, "low water" silicone materials do not include appreciable amounts of hydrophilic monomers and/or internal wetting agents (i.e. typically less than 5 to 10 weight percent). As such, low water silicone materials, as their name suggest, do not absorb or retain appreciable amounts of water, e.g. less than about 5 weight percent, and more typically less than about 1 or 2 weight percent.
It is important that low water silicone compositions, especially for soft contact lens applications, have desirable oxygen permeability, modulus (Young's modulus of elasticity) and elastic recovery. This is particularly important in the formation of soft contact lenses, as the modulus and elastic recovery of the lens material can have a significant impact upon lens "comfort." Lenses possessing high modulus often have a perceived stiffniess; lenses with too high elastic recovery can exhibit a "suction cup" effect on the eye.
One class of silicone monomers commonly used in silicone-containing contact lens materials are bulky polysiloxanylalkyl methacrylates, e.g. methacryloxypropyl tris (trimethylsiloxy) silane (commonly referred to as "TRIS"). Since these "bulky" monomers contain a large amount of silicon per molecule, they are useful for increasing oxygen permeability of materials, a desirable property for contact lens and other biomedical device applications. However, although TRIS is known to reduce the modulus of some silicone hydrogels, i.e. polyurethane-polysiloxane hydrogel compositions, (see for example; Lai, Yu Chin, The Role of Bulky Polysiloxanylalkyl Methacrylates in Polyurethane-polysiloxane Hydrogels, Proceedings of the American Chemical Society Division of Polymeric Materials: Science and Engineering, Vol. 72, pg. 118-119, (1995)), the use of TRIS monomeric units within "low water" silicone compositions generally increases the modulus of the resulting material. As such, TRIS monomeric units are not generally helpful in reducing the modulus of low water silicone materials.
In summary, low water silicone materials are sought for soft contact lens or intraocular lens applications which possess relatively low modulus, e.g. below 300 g/mm.sup.2 and preferably from 20 g/mm.sup.2 to about 150 g/mm.sup.2, and with a suitable elastic recovery rate. Furthermore, for such applications, the materials must be optically clear, manufacturable (e.g., capable of being molded or machined), have acceptable oxygen permeability, biocompatibility and resist deposit formation. Low water materials are desired which can be easily synthesized, purified, and analyzed.